Fix checkstyle issue

Signed-off-by: Alan Zhang <shuai.xyz@gmail.com>

Author: alnzng

samza-core/src/test/java/org/apache/samza/container/grouper/task/TestTaskPartitionAssignmentManager.java

@@ -20,7 +20,6 @@
 
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
-import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 import org.apache.samza.config.Config;